The present invention relates to a skew compensation device for compensating for a skew between a data signal and a clock signal.
It is required that in various data receiving components including D latches, data signals are determined before more than the definite time period (called the setup time period) from a clocking edge of the clock signal. Even when data signals are generated in such a way as to secure a given setup time period for clock signals, there is produced a skew (a "deviation" in time) between the clock signal and a data signal if there is a difference in propagation delay time between these signals. As a result, phase relationship problems may occur between clock and data signals received by the data receiving component. When data are transmitted at a high speed, the data are likely to be received with errors.
Hamamoto and the others disclosed a skew compensation device in a paper entitled "400 MHz Random Column Operating SDRAM Techniques with Self Skew Compensation," 1997 Symposium on VLSI Circuits, Digest of Technical Papers, pp. 105-106. This skew compensation device has the following components: a fixed delay line for generating a delayed data signal which lags an input data signal by a fixed amount of delay; a variable delay line for generating a delayed clock signal which lags the input clock signal by a variable amount of delay; and a phase comparator for comparing a transition of the delayed data signal with a phase of the delayed clock signal and for controlling the delay amount of the variable delay line so that the delayed data signal transition is substantially phase matched with a clocking edge of the delayed clock signal. The skew compensation device outputs the delayed clock signal as a clock signal for latching the input data signal. This technique secures a data setup time period coincident with the delay amount of the fixed delay line.
A typical skew compensation device operates only in setup mode when power is turned on and "sleeps" in normal operating mode, resulting in producing some problems. One problem is that it is impossible for such a skew compensation device to perform, in a normal operating mode, skew compensation operations according to environmental changes such as an increase in temperature. The reason is that if the skew compensation device is forced to uninterruptedly operate, the phase comparator will malfunction when both of the input data signal and the delayed data signal undergo no transition therefore holding respective constant logical levels.